KineticsFactors affecting reactions rates, collision theory, relative rates, differential rate law,

concentration and rate

Notes- AP Chem

What is Chemical Kinetics?

• Chemical kinetics-the study of the speed or rate of a reaction under various conditions.

Things to consider as we begin our study of kinetics

• Spontaneity - a spontaneous reaction does NOT imply a rapid reaction. – The changing of diamond into graphite is

spontaneous but so slow that it is not detectable even in a lifetime.

• A mechanism is a sequence of events at the molecular level that controls the speed and outcome of the reaction.

FACTORS THAT AFFECT REACTION RATES

1. Nature of the reactants--Some reactant molecules react in a hurry, others react very slowly.

Pointers:

• Physical state- – gasoline (l) vs. gasoline (g) ;

– K2SO4(s) + Ba(NO3)2(s) no rxn.; while both of these in the aqueous state react.

Chemical identity – – What is reacting? – Usually ions of opposite charge react very rapidly. – Usually, the more bonds between reacting atoms in a

molecule, the slower the reaction rate. – Substances with strong bonds (larger bond energies)

will react much more slowly. – Examples:

• metallic sodium reacts much faster with water than metallic calcium.

• Oxidation of methane can be increased with an increase in temperature; photosynthesis is very slow and changes very little with an increase in temperature.

• 2. Concentration of reactants--more molecules results in more collisions; more collisions results in more “successful” collisions; more “successful” collisions means increased rate of reaction

• 3. Temperature--heat ‘em up & speed ‘em up; the faster they move, the more likely they are to collide.

• An increase in temperature produces more successful collisions that are able to overcome the needed activation energy, therefore, a general increase in reaction rate with increasing temperature.

• In fact, a general rule of thumb is that a 10C increase in temperature will double the reaction rate.

• * This actually depends on the magnitude of the Ea (activation energy) and the temperature range.

• 4. Catalysts--accelerate chemical reactions but are not themselves transformed. – Present at the beginning of the reaction and are present at the

end in the same form.

• A catalyst is a substance that changes the rate of reaction by altering the reaction pathway. Most catalysts work by lowering the activation energy needed for the reaction to proceed, therefore, more collisions are successful and the reaction rate is increased.

• Biological catalysts are proteins called enzymes.

• Remember! The catalyst is not part of the chemical reaction and is not used up during the reaction.* (May be homogeneous or heterogeneous catalysts.) – Ex. H2O2 decomposes relatively slowly into H2O and O2;

however; exposure to light accelerates this process AND with the help of MnO2, it goes extremely FAST!!

• Note: A catalyst lowers the activation energy barrier. Therefore, the forward and reverse reactions are both accelerated to the same degree.

• * (Some homogeneous catalysts actually appear in the rate law because their concentration affects the reaction. Ex. NO catalyzing O3 )

5.Surface area of reactants--exposed surfaces affect speed.

• Except for substances in the gaseous state or solution, reactions occur at the boundary, or interface, between two phases.

• The greater surface area exposed, the greater chance of collisions between particles, hence, the reaction should proceed at a much faster rate. Ex. coal dust is very explosive as opposed to a piece of charcoal. Solutions are ultimate exposure!

• Adding an inert gas has NO EFFECT on the rate (or equilibrium for that matter) of the reaction.

THE COLLISION THEORY OF REACTION RATES

 • Particles must collide.• Only two particles may collide at one time. • Proper orientation of colliding molecules so that atoms in

the can come in contact with each other to become products.

• The collision must occur with enough energy to overcome the electron/electron repulsion of the valence shell electrons between the reacting species and must have enough energy to transform translational energy into vibrational energy in order to penetrate into each other so that the electrons can rearrange and form new bonds.

• This new collision product is at the peak of the activation energy hump and is called the activated complex or the transition state. At this point, the activated complex can still either fall to reactants or to products.

• With all of these criteria met, the reaction may proceed in the forward direction. Amazing that we have reactions occurring at all!

CHEMICAL REACTION RATES

• The speed of a reaction is expressed in terms of its “rate”, some measurable quantity is changing with time.

• The rate of a chemical reaction is measured by the decrease in concentration of a reactant or an increase in concentration of a product in a unit of time.

• Rate = change in concentration of a species

time interval •  When writing rate expressions, they can

be written in terms of reactant disappearance or product appearance.

Instantaneous Rate

• Rate is not constant, it changes with time. Graphing the data of an experiment will show an average rate of reaction. You can find the instantaneous rate by computing the slope of a straight line tangent to the curve at that time.

• reaction rate--expressed as the Δ in concentration of a reagent per unit time or Δ[A]/Δt

• focus either on the disappearance of reactants or the appearance of products

• rate of Δ of a reactant is always negative• rate of Δ of a product is always positive•  

• Consider: 2 NO2(g) O 2(g) + 2 NO(g)

• Oxygen can appear only half as rapidly as the nitrogen dioxide disappears while NO appears twice as fast as oxygen appears (look at the coefficients!)

• Note that the rate is NOT constant but decreases with time. The rates given below are average rates.

−Δ [NO2] (× 10-5)

Δt

Time period (s)

4.2

0 50

2.8

50 100

2.0

100 150

1.4

150 200

1.0

200 250

• Examine the data table on the previous slide and calculate the AVERAGE rate at which [NO2] changes in the first 50.0 seconds: (Remember the square brackets are shorthand for molarity!)

•  

• RATE = −Δ [NO2] = −[.0079]−[0.0100]

Δt 50.0 s• = −[−4.2 × 10−5 mol/L sec]• = 4.2 × 10−5 mol/L sec or M s−1

RELATIVE RATES

• We can consider the appearance of products along with the disappearance of reactants. The reactant’s concentration is declining, the products is increasing. Respect the algebraic sign AND respect the stoichiometry.

• [Divide the rate of change in concentration of each reactant by its stoichiometric coefficient in the balanced chem. eqn. and this is foolproof and a breeze!]

• Thus.....

Rate of reaction = - 1Δ[NO2] = 1 Δ[NO] = Δ [ O2]

• 2 Δtime 2 Δtime Δtime

• Of course you can change these once the ratio is set. You might prefer -1 : +1 : +2

• Relative Rates from the balanced equation:

• Using the coefficients from the balanced equation, you should be able to give relative rates. For example:

4 PH3 (g) P4(g) + 6 H2(g)

Initial rate rxn. =

time

PH ][

4

1 3 =

time

P ][ 4 =

time

H ][

6

1 2

• Exercise • What are the relative rates of change in

concentration of the products and reactant in the decomposition of nitrosyl chloride, NOCl?

• 2 NOCl (g) 2 NO(g) + Cl2(g)

•  

Differential RATE LAW: AN INTRODUCTION 

• Reactions are reversible. So far, we’ve only considered the forward reaction. The reverse is equally important. When the rate of the forward = the rate of the reverse we have EQUILIBRIUM! To avoid this complication we will discuss reactions soon after mixing--initial reactions rates, and not worry about the buildup of products and how that starts up the reverse reaction.

•  

• initial reaction rates--begin with pure reactants, mix thoroughly, then measure speed of rxn. over time

• The presence of products can alter results dramatically and lead to confusing results. We’ll be talking initial reaction rates throughout our discussions!

• The (differential) rate expression or rate law expression is the relation between reaction rate and the concentrations of reactants given by a mathematical equation.

Rate = k[NO2]n =

t

NO ][ 2

CONCENTRATION AND REACTION RATE DATA

• THE DIFFERENTIAL RATE LAW OR RATE EXPRESSSION: Rates generally depend on reactant concentrations.

• To find the exact relation between rate and concentration, we must conduct experiments and collect information

• We measure the way in which the starting concentrations affect the rate of the reaction

– Examine data given to you to determine how changing the concentrations changes the rate

– Use this information to write the RATE LAW or rate expression for the reaction • shows the mathematical relationship between

concentration and rate

• For the generic reaction

C

aA + bB xX

Where C is a catalyst, the rate expression will always have the form Initial Rxn Rate=k [A]m [B]n [C]p

Initial Rxn Rate=k [A]m [B]n [C]p

• k = rate constant• [A] = concentration of reactant A• [B] = concentration of reactant B • [C] = concentration of the catalyst—won’t see this too

often in AP• m = order of reaction for reactant A• n = order of reaction for reactant B• p = order of reaction for the catalyst C• Exponents can be zero, whole numbers or fractions AND

MUST BE DETERMINED BY EXPERIMENTATION!!

The Rate constant, k• depends on specific reaction and temperature• does not change with changing concentrations• Units are different depending on the overall order

of the reaction, and are important!– Units of rate constant = units of rate/ (units of

concentration)overall reaction order

reaction orders: m, n, and p– Order wrt (with respect to) a certain reactant is the exponenet on

its concentration term in the rate expression– must be determined experimentally

– they do not necessarily correspond to coefficients– gives relationship between concentration of reactant and rate– If the concentration of a reactant has no effect on rate, it has a

reaction order of zero and is omitted from the rate law• Reactions orders of 0, 1, or 2 are common, although it is

possible to have fractional or even negative orders– the overall reaction order or order of the reaction is the sum of

all the reaction orders in the rate law

• Zero order: The change in concentration of reactant has no effect on the rate. These are not very common. General form of rate equation: Rate = k

• First order: Rate is directly proportional to the reactants concentration; doubling [rxt], doubles rate. These are very common! Nuclear decay reactions usually fit into this category. General form of rate equation: Rate = k [A]1 = k[A]

• Second order: Rate is quadrupled when [rxt] is doubled and increases by a factor of 9 when [rxt] is tripled etc. These are common, particularly in gas-phase reactions. General form of rate equation: Rate = k [A]2 or Rate = k[A]1[B]1 which has an overall order of two (second order).

• Fractional orders are rare, but do exist!

• If our general rate expression is: Rate = k[A]m[B]n

• If m = 0 ; reaction is zero order with respect to A • If m = 1 ; reaction is 1st order with respect to A• If m = 2 ; reaction is 2nd order with respect to A• If n = 0 ; reaction is zero order with respect to B• If n = 1 ; reaction is 1st order with respect to B• If n = 2 ; reaction is 2nd order with respect to B•  • Adding the orders of each reactant gives the overall order of the

reaction.

Analyze this rate law…

A + B C + DRate = k [A ]2 [B]

• holding [B] constant, – If we increase [A] by a factor of 2, then rate will increase by a

factor of 4– If we increase [A] by a factor of 3, then rate will increase by a

factor of 9• Holding [A] constant,

– If we increase [B] by a factor of 2, then rate will increase by a factor of 2

– If we increase [B] by a factor of 3, then rate will increase by a factor of 3

Determining a rate law from experimental data…

• Given the following data:

Experiment Number

Initial Rate mol/(L hr)

Initial concentration

[A]o

Initial concentration

[B]o

1

0.50 × 10−2

0.50

0.20

2

0.50 × 10−2

0.75

0.20

3

0.50 × 10−2

1.00

0.20

4

1.00 × 10−2

0.50

0.40

5

1.50 × 10−2

0.50

0.60

METHOD 1: APPLY “TABLE LOGIC”

1. Look for two trials where the concentration of a reactant was held constant.

2. Next, focus on the other reactant. Ask yourself how it’s concentration changed for the same two trials. Was it doubled? Was it tripled? Was it halved?

3. Once you have determined the factor by which the concentration of the other reactant was changed, determine how that affected the rate for those same two trials. Expect easy math! Did changing the concentration have zero effect on the rate? If so, then it is zero order. Did the rate double when the concentration of the other reactant was doubled? If so, it is first order. Did the rate quadruple as a result of the reactant’s concentration doubling? If so, it is second order. Did the rate increase by a factor of eight? If so, it is third order.

THINK of the concentration doubling as the number “two”. So, rate = k[reactant] becomes, rate = k[2]m and you are trying to determine the value of m. If the rate doubled think (2 rate) = k[2]m and more simply 2 = [2]m, so m = 1. If the rate quadrupled, then think simply, 4 = [2]m and m must equal 2 to make that a true statement, and so on…

4. Finally, examine the data table again. This time look for trials where the concentration of the reactant you just determined the order for is held constant and repeat steps 1-3 above.

•  

• For our example data table, the rate stays the same regardless of the concentration of [A], therefore,

• it is zero order with respect to A. However, the rate doubles with a doubling of [B] and triples with a tripling of [B]. This indicates the rate is first order with respect to [B].

•Summary: Initial reaction rate = k[A]o

o[B]o1 = k[B]o

1 = k[B]

• The overall reaction rate order = 1 + 0 = 1st order overall.

• Now. . . • Use a set of the data to calculate k:• 0.0050 mol/(Lhr) = k[0.20 mol/L]1

• k = 2.5 × 10−2 /hr or 2.5 × 10−2 hr−1 •  • You should get the same value with any line of data!

METHOD 2:APPLY “ALGEBRA”

• Ugly algebraic method is sometimes necessary:

• rate 1 = k[reactant]m [reactant]n rate 2 k[reactant]m [reactant]n 

• Select a trial where one reactant concentration is held constant SO THAT IT CANCELS;

• the k’s will also cancel • Using trials 1 & 4:• 0.50 × 10-2 = k [0.50]m [0.20]n so…. ½ = [ ½ ]n and n must be

ONE to make that true! It• 1.00 × 10-2 k [0.50]m [0.40]n

• It’s just the long-hand version of table logic.

•  

This may be useful:

(factor concentration increased by)n =

(factor rate will increase by)

• If you have a case where one reactant is never held constant, then you can either add an “expect” column to the table since you usually can determine the order of at least one reactant and can predict the “expected” change in the rate. Then proceed as usual and compare your “expected rate” to the actual rate for doubling the concentration of the other reactant. OR just plug into ugly algebra, just use the method that makes you both quick and accurate!

•  

ExampleGiven data:

BrO3- + 5Br- + 6H+ 3Br2 + 3H2O

Mixture [BrO3-] [Br-] [H+] Rate

(M/time)A 0.0050 0.025 0.030 10B 0.010 0.025 0.030 20C 0.010 0.050 0.030 40D 0.010 0.050 0.060 160

Determine the rate law for this reaction (in other words, figure out the orders of your reactants)

• Exercise • In the following reaction, a Co-Cl bond is replaced by a Co-OH2

bond.• [Co(NH3)5Cl]+2 + H2O [Co(NH3)5H2O]+3 + Cl

• Initial rate = k{[Co(NH3)5Cl]+2}m

• Using the data below, find the value of m in the rate expression and calculate the value of k.

• Exp. Initial Concentration Initial rate• of [Co(NH3)5Cl]+2 mol/(L min)

• (mol/L) • 1 1.0 × 10-3 1.3 × 10-7

• 2 2.0 × 10-3 2.6 × 10-7

• 3 3.0 × 10-3 3.9 × 10-7

• 4 1.0 × 10-3 1.3 × 10-7

• Exercise • The reaction between bromate ions and bromide ions in acidic aqueous solution is

given by the equation• BrO3

- (aq) + 5 Br – (aq) + 6 H+ (aq) 3 Br2 (l) + 3 H2O (l)

• The table below gives the results of four experiments. Using these data, determine the orders for all three reactants, the overall reaction order, and the value of the rate constant. What is the value of k? What are the units of k?

•  • Exp Initial [BrO3

-] Initial [Br –] Initial [H+] Measured initial rate (mol/Ls)

• 1 0.10 0.10 0.10 8.0 x 10-4

• 2 0.20 0.10 0.10 1.6 x 10-3

• 3 0.20 0.20 0.10 3.2 x 10-3

• 4 0.10 0.10 0.20 3.2 x 10-3

•